Magnetic metal power and method of producing the powder

ABSTRACT

A magnetic metal powder having fluidity is provided which is composed of FePt nanoparticles synthesized by the polyol synthesis method that possess fct (face-centered tetragonal) structure and exhibit crystal magnetic anisotropy from immediately after synthesis. Specifically, there is provided a magnetic metal powder having fluidity which is composed of magnetic metal particles whose main components and the contents thereof are represented by the following general formula (1): 
       [T X M 1−X ] Y Z 1−Y   (1), 
     where T is one or both of Fe and Co, M is one or both of Pt and Pd, Z is at least one member selected from the group composed of Ag, Cu, Bi, Sb, Pb and Sn, X represents 0.3˜0.7, and Y represents 0.7˜1.0, the balance being impurities unavoidably incorporated during production, which magnetic metal powder has a volumetric ratio of ferromagnetic structure (face-centered tetragonal ratio) as measured by Mossbauer spectroscopy in the range of 10˜100%, saturation magnetization σs of 20 emu/g or greater, and average primary particle diameter by transmission electron microscopic observation (TEM) of 30 nm or less.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a magnetic metal powder that can be used inhigh-density magnetic recording media, nanoscale electronics, permanentmagnet materials, biomolecular labeling agents, drug carriers and thelike and to a method of producing the magnetic metal powder. Although,strictly speaking, the magnetic metal powder of the present invention iscomposed of metal particles characterized by general formula (1)discussed later, owing to the fact that it is typically an FePt-systemalloy in which T=Fe, M=Pt, the particles of the magnetic metal powderare referred to in this specification simply by the terms “FePtparticles” and “FePt nanoparticles” as being representative of suchmagnetic metal particles.

2. Background Art

In order to increase the recording density of a high-density magneticrecording medium, it is necessary to reduce the size of the basic unitfor recording. However, conventional media using sputter-formed filmsare approaching the limit to which recording density can be increasedbecause of problems in such areas as thermal fluctuation, grain refiningand variance of crystal grain size. Recently, therefore, attention hasfocused on FePt-system magnetic metal nanoparticles as a high-densitymagnetic recording medium material that is not susceptible to thermalfluctuation, has high anisotropy, and exhibits strong coercivity.

Japanese Patent No. 3258295 (JPA No. 2000-54012; herein after calledReference 1) teaches a method of producing such magnetic metalnanoparticles, namely a method producing FePt alloy particles in amonodispersed state by conducting thermal decomposition reaction of ironpentacarbonyl simultaneously with reduction of platinum (II)acetylacetonate by the action of polyol. On the other hand, in Journalof Applied Physics, Vol. 87, No. 9, 1 May 2000, p. 5615-5617 (Reference2), a method of reducing metal ions using boron hydride is reported inwhich-the reaction site is a W/O (water/oil) type reversed micelleutilizing an octane oil phase and CTAB (cetyl trimethyl ammoniumbromide) as surface active agent.

The crystal structure of the FePt particles obtained by these methods isa disordered fcc (face-centered cubic) structure, so that the particlesexhibit superparamagnetism on the nano order. If they are to be used asferromagnetic particles, therefore, it is necessary to carry out heattreatment for transforming the crystal structure to an L1 ₀ orderedphase that is fct (face-centered tetragonal) phase.

The heat treatment has to be conducted at or above the crystal structuretransition temperature (Tt) between the disordered phase and orderedphase and is ordinarily conducted at a high temperature of 450° C. orhigher. At the time of this heat treatment, the granularity distributionbroadens owing to grain enlargement caused by heat-induced coalescenceamong the particles. As a result, the particles come to be present in amixture of single domain and multi-domain structures that makes themunsuitable for a high-density magnetic recording media. Therefore, inorder to obtain FePt particles having ferromagnetism while maintainingtheir grain diameter immediately after synthesis, it is effective tocoat the particles with a protective agent for preventing inter-particlecoalescence or to lower Tt by some method so that the heat treatment canbe conducted at a low temperature.

Denshi Zairyo (Electronic Materials) January 2002, p 61-67 (Reference 3)reports that addition of elements such as Ag, Cu, Sb, Bi and Pb duringsynthesis of FePt particles by the polyol method makes it possible tolower the crystal structure transition temperature (Tt) between fccstructure and fct structure.

The FePt particles obtained by the methods of References 1-3 have anonmagnetic fcc (face-centered cubic) structure immediately followingthe reaction and cannot be used unmodified as magnetic particles for amagnetic recording medium. This makes it necessary to subject them toheat treatment at a temperature equal to or higher than the fct crystalstructure transition temperature (Tt) so as to transform them to an fct(face-centered tetragonal) structure exhibiting magnetism.

The crystal structure transition temperature of the FePt particlesobtained by these methods is around 450° C., so that heat treatment at atemperature of 450° C. or higher is required for transition to the fctstructure. However, when the aggregate (powder) composed of these FePtparticles is heated to a temperature of 450° C. or higher, the metalparticles enlarge through coalescence. Therefore, even though an fctstructure can be obtained, the FePt particles do not assume ananoparticle morphology suitable for use in a high-density recordingmedium, and since particle coalescence is usually not uniform, therearises a grain distribution that broadens the range of magneticcharacteristic distribution to pose problems from the practicalviewpoint.

In order to prevent particle enlargement by heat-induced inter-particlecoalescence, the heat treatment must be carried out in a state with theposition of the individual particles fixed at prescribed spacing (withthe particles fixed at prescribed locations on a substrate, forexample). However, realization of such heat treatment requires use of asophisticated technology for precisely positioning the particles in anorderly fashion. While this may be technically feasible, a still bettersolution would be for the FePt particles to possess fct structure fromimmediately after synthesis, because this would offer the considerablemerit of eliminating the need for such heat treatment or at leastsimplify it (by enabling use of a low heat treatment temperature, forexample).

Although it has been reported that the Tt temperature is lowered by theeffect of elements added to the FePt alloy, the need for heat treatmentafter reaction still remains, namely a heat treatment temperature of atleast 300° C. is required for transition to fct structure, which limitsthe materials that can be used for the substrate/base and causes anumber of other inconveniences. Moreover, in the case where FePtparticles of fcc structure are transformed to fct structure by heattreatment conducted after the particles are positioned on a substrate,the particles assume uniaxial crystal magnetic anisotropy during theheat treatment process. The direction of this axis is random when viewedwith respect to the substrate, for example. Alignment of the axis in acertain direction with respect to the substrate requires the heattreatment and the like to be carried out in a magnetic field. This isdifficult in actual practice. Owing to the fact that particles heattreated on a substrate are adhered to the substrate by sintering or thelike, it is extremely difficult to rearrange them as powder on anothersubstrate or base. On the other hand, if the particles should haveuniaxial crystal magnetic anisotropy from the start and should furtherbe in the state of a powder enabling the particles to flow freely, itwould be easily possible to disperse the particles in resin anduniaxially align them with respect to a substrate at normal temperatureby applying a conventional technology used for drying a coating-typemagnetic recording medium.

SUMMARY OF THE INVENTION

An object of the present invention is therefore to provide a magneticmetal powder whose particles have fct structure from immediately afterbeing obtained by a reaction and therefore can be obtained as magneticmetal powder having fluidity that eliminates or mitigates the need forheat treatment.

The inventor succeeded in obtaining a magnetic metal powder composed ofnanoparticles having fct structure at the time of completion ofsynthesis reaction.

Specifically, I accomplished the present invention, which provides amagnetic metal powder having fluidity which is composed of magneticmetal particles whose main components and the contents thereof arerepresented by the following general formula (1):

[T_(X)M_(1−X)]_(Y)Z_(1−Y)  (1),

where T is one or both of Fe and Co, M is one or both of Pt and Pd, Z isat least one member selected from the group composed of Ag, Cu, Bi, Sb,Pb and Sn, X represents 0.3˜0.7, and Y represents 0.7˜1.0, the balancebeing impurities unavoidably incorporated during production, whichmagnetic metal powder has a volumetric ratio of ferromagnetic structure(face-centered tetragonal ratio) as measured by Mossbauer spectroscopyin the range of 10˜100%, saturation magnetization σs of 20 emu/g orgreater, and average primary particle diameter by transmission electronmicroscopic observation (TEM) of 30 nm or less. The magnetic metalpowder according to the present invention preferably has a magneticanisotropy Hk by magnetic torque measurement of 10 kOe or greater, morepreferably 12.4 kOe or greater and preferably has an average primaryparticle diameter of 20 nm or less, more preferably 10 nm or less.

The present invention also provides a method of producing a magneticmetal powder comprising a step of producing a magnetic metal powdercomposed of magnetic metal particles of a substance represented by thefollowing general formula (1):

[T_(X)M_(1−X)]_(Y)Z_(1−Y)  (1),

where T is one or both of Fe and Co, M is one or both of Pt and Pd, Z isat least one member selected from the group composed of Ag, Cu, Bi, Sb,Pb and Sn, X represents 0.3˜0.7, and Y represents 0.7˜1.0, the balancebeing impurities unavoidably incorporated during production, in whichstep metal salts containing the T and M components and if required the Zcomponent are dissolved in a solvent composed of a polyalcohol or aderivative of a polyalcohol and having a boiling point of 270° C. orhigher, holding the solution at a temperature of 270° C. or higher toreduce the metal salts with the polyalcohol or polyalcohol derivativeand synthesize particles of the substance by the reduction, at whichtime the particles synthesized by the reduction, in their state assynthesized, have a volumetric ratio of ferromagnetic structure(face-centered tetragonal ratio) as measured by Mossbauer spectroscopyin the range of 10˜100%, saturation magnetization σs of 20 emu/g orgreater, and average primary particle diameter by transmission electronmicroscopic (TEM) observation of 30 nm or less.

The polyalcohol can be one or both of triethylene glycol andtetraethylene glycol and the salts of the T, M and Z components can beacetylacetonates of these components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows transmission electron micrographs of primary and secondaryparticles of a magnetic metal powder in accordance with the presentinvention.

FIG. 2 shows an example of the results of Mossbauer spectroscopy carriedout on a magnetic metal powder according to the present invention.

FIG. 3 shows a comparison of X-ray diffraction charts obtained for amagnetic metal powder according to the present invention and a magneticmetal powder of a comparative example.

FIG. 4 shows an example of the results of Mossbauer spectroscopy carriedout on a magnetic metal powder of a comparative example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Rather than transforming synthesized FePt nanoparticles to fct structureby heat treatment as is in done in References 1-3, a goal was set toproduce FePt nanoparticles that had an fct structure from the time theywere synthesized. To this end, an attempt was made to synthesize FePtnanoparticles by using a polyalcohol of the highest possible boilingpoint as reducing agent. As a result, it was found that when apolyalcohol having a boiling point of 270° C. or higher is used at areaction temperature of 270° C. or higher to reduce Fe ions and Pt ionsin the polyalcohol under circulation, FePt nanoparticles including fctstructure are directly synthesized.

The polyalcohol is preferably triethylene glycol or tetraethyleneglycol. However, the invention is not limited to use of thesepolyalcohols but can use any of various polyalcohols and polyalcoholderivatives having a boiling point of 270° C. or higher. Use of ethyleneglycol is undesirable because of its low boiling point of 197° C. (seeComparative Example 2, for instance). The Fe ions and Pt ions in thepolyalcohol are typically supplied in the form of iron (III)acetylacetonate and platinum (II) acetylacetonate.

The particulars defined by the present invention will now be explained.

Metal Components

The main components of the magnetic metal particles of the presentinvention, and the contents thereof, are represented by the followinggeneral formula (1):

[T_(X)M_(1−X)]_(Y)Z_(1−Y)  (1),

where T is one or both of Fe and Co, M is one or both of Pt and Pd, Z isat least one member selected from the group composed of Ag, Cu, Bi, Sb,Pb and Sn.

T and M are typically Fe and Pt. Although X=0.5 is ideal for forming aface-centered tetragonal structure, a 10˜100% face-centered tetragonalmetal structure can be obtained in the range of X of 0.3˜0.7. Althoughthe Z component can lower the crystal structure transition temperature(Tt) between fcc structure and fct structure in the case of synthesizingFePt particles by the polyol method, it need not be contained in somecases. Specifically, the optimum value of Y, although differingdepending on the type of Z, falls in the range of 0.7˜1.0. A value of Yof less than 0.7 is undesirable because the resulting presence ofexcessive Z sharply depresses magnetic characteristics by hindering theemergence of fct structure. Analysis of the composition of the magneticmetal particles according to the present invention can be carried out byEDX (energy dispersive X-ray) measurement. Although the magnetic metalpowder is ideally made up of metal particles having the compositionrepresented by Equation (1), presence of impurities unavoidablyincorporated in the course of production can be tolerated.

In light of the fact that, as explained in the foregoing, thecomposition of the magnetic metal particles according to the presentinvention is typically FePt, the ensuing explanation will be made usingFePt particles as an example. In this specification, the term “FePtparticles” is, by definition, to be construed as referring to magneticmetal particles represented by Equation (1).

Face-Centered Tetragonal Structure Ratio

The FePt particle powder according to the present invention has avolumetric ratio of ferromagnetic structure (face-centered tetragonalratio) as measured by Mossbauer spectroscopy in the range of 10˜100%.The breakdown of metallic phases (the relative amounts of differentcrystal structures) present in a metallographic structure is generallydetermined by comparing X-ray diffraction peak intensities. However, itis difficult to quantify the metallic phases of the invention FePt alloyfrom these peaks only, because the X-ray diffraction patterns of the fcc(face-centered cubic) structure and the fct (face-centered tetragonal)structure are nearly identical and the intensities of the (001) and(110) reflections obtained only from the fct structure are very weak.

However, it is possible to calculate the volumetric ratio of the fctstructure by analyzing the volumetric ratio of the ferromagneticstructure measured with respect to the FePt alloy by Mossbauerspectroscopy. In this invention, therefore, the volumetric ratio of thefct structure of the FePt particles is determined by analyzing thevolumetric ratio of the ferromagnetic structure determined by Mossbauerspectroscopic measurement of Fe atoms, i.e., by calculating the ratio ofthe number of Fe atoms determined to be under magnetic order byMossbauer spectroscopy measurement of Fe atoms, and the so-obtainedratio is defined as the volumetric ratio or the fct structure.

When the volumetric ratio (vol. %) of the fct structure, i.e., theface-centered tetragonal structure, is less than 10 vol. %, the magneticanisotropy is small and the coercivity and thermal stability required bya magnetic recording material cannot be obtained. When the magneticanisotropy is too large, the resulting excessively high coercivity makesthe product difficult to use as a material for a magnetic recordingmedium and rather makes it suitable for use as a material for strongpermanent magnets. The ratio of the face-centered tetragonal (fct)structure of the invention magnetic metal particles is therefore definedas 10˜100%.

Grain Diameter

The FePt particle powder according to the present invention has anaverage primary particle diameter by transmission electron microscopicobservation (TEM) of 30 nm or less, preferably 20 nm or less, morepreferably 10 nm or less. A primary particle is defined as the smallestunit particle incapable of further division. At the time of synthesis,the fct structure FePt particle powder synthesized in accordance withthe present invention generally has present therein clusters consistingof many primary particles that form owing to the static magnetic fieldacting among the primary particles. In other words, the primaryparticles tend to collect into clusters so that a state in whichnumerous clusters are distributed throughout the powder is likely tooccur. Such a cluster formed of numerous primary particles is called asecondary particle. Although the grain diameter of the secondaryparticles differs considerably depending on the conditions of thesynthesis reaction, it is about 100 μm in the Examples set out below. Atany rate, irrespective of whether or not such secondary particles form,the powder nevertheless has fluidity as a whole.

Magnetic Characteristics

Since the FePt particle powder according to the present invention hasfct structure from the time of synthesis (without being subjected toheat treatment), it exhibits a magnetic anisotropy Hk by magnetic torquemeasurement of 10 kOe or greater, more preferably 12.4 kOe or greater.Magnetic anisotropy Hk is a magnetic property directly related toanisotropy constant Ku, as shown by Equation (2), in which Ms representssaturation magnetization (emu/g):

Hk=2 Ku/Ms  (2)

Thermal stability factor can be calculated from the anisotropy constantKu using Equation (3):

Thermal Stability Factor=KuV/κT  (3),

where V is particle volume, κ is Boltzmanm constant and T istemperature. The thermal stability factor is an index of magneticrecording medium thermal stability. A magnetic metal powder having athermal stability factor value of less than 60 is said to be unsuitablefor practical use (Y. Hosoe et al, Journal of Magnetics Society ofJapan, vol. 22, No. 12, 1998).

When the thermal stability factor is small, recorded signals vanishspontaneously. In order to achieve high recording density, since it isimportant for this that the magnetic particles to be sufficiently smallrelative to the recording wavelength, volume V in Equation 3 must bemade considerably small. From this it follows that for making thethermal stability factor large it is necessary to make the anisotropyconstant Ku large. For Ku to be large, it is important in light ofEquation (2) for the magnetic anisotropy Hk and saturation magnetizationMs to be large, and an actual magnetic recording magnetic powderpreferably has a magnetic anisotropy Hk at room temperature of 10 kOe orgreater and a saturation magnetization σs at room temperature of 20emu/g or greater. This is because at Hk of less than 10 kOe and σs ofless than 20 emu/g the thermal stability of the magnetic particles is somarkedly poor as to make the magnetic metal powder unstable for magneticrecording applications. The FePt particle powder according to thepresent invention exhibits a magnetic anisotropy Hk at room temperatureof 10 kOe or greater and a saturation magnetization σs at roomtemperature of 20 emu/g or greater. As its magnetic particles aretherefore excellent in thermal stability, it is suitable as a magneticpowder for use in magnetic recording media.

The FePt particles of the powder at the time of synthesis may becomeinterconnected owing to the action of the static magnetic field, andwhen this happens, the resulting overall band-like morphology exhibitsshape magnetic anisotropy. When coercivity is measured, therefore, it ispossible that the measurement will include not only the coercivity owingto the crystal magnetic anisotropy of the FePt particles but also thecoercivity owing to this shape anisotropy. The coercivity measured whenthe particles are separated from one another, i.e., when they are fixedat spaced apart locations, is the true coercivity of the FePt particleshaving fct structure according to the present invention.

Crystal Structure Transition Starting Temperature

The FePt particle powder according to the present invention is composedof 10˜100 vol. % of fct structure and the remainder of fcc structure.The transition starting temperature for transforming the remaining fccstructure to fct structure is lower than the transition startingtemperature (450° C.) for transforming completely fcc structure FePtparticles to fct structure as in References 1 and 2. In Example 1 setout later, no transition starting temperature was observed because thestructure of the magnetic metal powder was almost totally accounted forby fct structure. The transition starting temperature can be ascertainedfrom the exothermic peak measured using a differential scanningcalorimeter.

Production Method

In conventional FePt particles production methods (such as described inReferences 1 and 2), the obtained FePt particles have fcc (face-centeredcubic) structure, which is a disordered phase. In order to transformtheir phase to L1 ₀ orderly phase (fct structure) manifestingferromagnetism they have to be heat treated at a temperature equal to orhigher than the crystal structure transition starting temperature (Tt)(450° C.). These conventional methods utilize thermal decomposition ofiron pentacarbonyl and metal ion reduction reaction by the strongreducing power of boron hydride. As such, they can be called productionmethods using fast reactions.

On the other hand, the inventor first learned that when both Fe and Ptare reduced from acetylacetonate complex by polyol, i.e., by a method inwhich the reaction speed is suppressed, it becomes possible to lower theFePt particle crystal structure transition starting temperature (Tt) toaround 310° C. The polyol they used at this stage of their research wasethylene glycol (boiling point: 197.6° C.) and the reaction temperaturewas 200° C. This case is set out below in Comparative Example 2.

Between Fe and Pt, Pt is the easier to reduce. Therefore, componentsegregation can be expected to occur inside the particles when thereaction speed of the production method is high. In this case, the heattreatment for transformation to the L1 ₀ ordered phase must be carriedout so as to diffuse Fe and Pt mutually in the particles. Tt thereforebecomes a high temperature of 450° C. or higher. In contrast, when thereaction speed is slowed down, component segregation does not readilyoccur in the particles, so that Tt can be lowered. The use of a Tt of280° C. in Comparative Example 2 can be considered to be for thisreason. However, the product of Comparative Example 2 has a low magneticanisotropy Hk of 3.7 kOe by magnetic torque measurement and, in itsstate as synthesized, can be assumed to have only a very small amount offct structure exhibiting magnetic anisotropy present. As such, it doesnot achieve the object of the invention set out earlier.

It was then discovered that when the reaction temperature was raised byusing a polyol having a higher boiling point than ethylene glycol, suchas triethylene glycol or tetraethylene glycol, a FePt particle powdercan be obtained that is rich in fct structure at the time of synthesis.It was further found regarding the relationship between highesttemperature reached by the solution (solvent) during the FePt particlesynthesis reaction and the magnetic anisotropy Hk that it is impossibleto satisfy the condition of Hk≧10.0 kOe when the highest temperature ofthe solvent is lower than 270° C.

Thus the inventor finding was that in a reduced reaction speed method ofreducing both Fe and Pt from acetylacetonate complexes using polyol, ifa solvent consisting of a polyalcohol or a derivative of a polyalcoholand having a boiling point of 270° C. or higher is used as the polyoland FePt particles are synthesized from the complexes by polyolreduction at a temperature of 270° C., a FePt particle powder possessingfluidity can be synthesized that in its state as synthesized has avolumetric ratio of ferromagnetic structure (face-centered tetragonalratio) as measured by Mossbauer spectroscopy in the range of 10˜100%,saturation magnetization σs of 20 emu/g or greater, and average primaryparticle diameter by transmission electron microscopic (TEM) observationof 30 nm or less. The boiling point of triethylene glycol andtetraethylene glycol is not lower than 270° C. Therefore, by using asolution containing these as solvent and making the highest temperatureof the solvent 270° C. or higher, FePt particles according to thepresent invention can be advantageously produced.

A dispersant can be incorporated in the reaction solution during thesynthesis reaction. The dispersant adheres to the particle surfaces andhelps to suppress aggregation among the particles. In addition, thegrain diameter of the synthesized FePt particles can be controlled bysuitably selecting the type and amount of the added dispersant.Dispersants suitable for use are ones that easily adhere to the metalparticle surfaces. These include surface active agents with a radicalincluding N atoms, specifically an amine, amide or azo radical, or anorganic molecule including either a thiol radical or a carboxyl radicalin its structure.

As explained earlier, it is important to control the speed of thissynthesis reaction. Also important is for the method used for this toalso control the metal concentration in the solvent. Specifically,supersaturation of the produced metal can be lowered by suppressing theconcentration of metal raw material, thereby reducing the rate ofnucleus generation and particle growth. If the mole ratio of all metalions contained in the polyol and the metal salts, i.e., the mole ratiopolyol/total metal ions, is 1000 or greater, the FePt particlesaccording the present invention can be advantageously produced.

The magnetic anisotropy Hk of the FePt particles obtained by thissynthesis reaction also varies with reaction time. Hk generallyincreases with increasing reaction time. Therefore, so as to obtain anadequately large Hk, the reaction time is set at 1 hr or more,preferably 2 hr or more, more preferably 3.5 hr or more.

The present invention will now be explained further with reference toworking examples and comparative examples.

EXAMPLE Example 1

Iron (III) acetylacetonate and platinum (II) acetylacetonate were addedto and dissolved in 100 mL of tetraethylene (boiling point: 327° C.) inan amount of 1.3 mmole/L each. The solution was transferred to a vesselequipped with a circulator. The vessel was placed on an oil-bath and thesolution was heated under stirring at 160 rpm while nitrogen gas wasbeing blown into the vessel as inert gas at a flow rate of 400 mL/min.Circulation was continued for 3.5 hr at a temperature of 320° C., atwhich time the reaction was terminated. The solution after terminationof the reaction was added with three-fold amount of methanol andcentrifuged. After removal of the supernatant liquid, the residual(particulate powder) was added with 100 mL of methanol and charged intoan ultrasonic washing vat. Following dispersion of the particulatepowder in the ultrasonic washing vat, the dispersed liquid wascentrifuged and the supernatant liquid was removed. The obtainedresidual (particulate powder) was thereafter subjected to two morecycles of the same washing operation consisting of methanol addition,dispersion in the ultrasonic washing vat and centrifugation. Thesubstance containing FePt nanoparticle powder obtained after finalremoval of supernatant liquid was subjected to the following tests.

FIG. 1 shows transmission electron micrographs of the obtained FePtnanoparticle powder. As can be seen from micrograph (A) in FIG. 1, theaverage grain diameter of the primary particles as measured from thismicrograph was about 5 nm. Further, as can be seen from micrograph (B),the primary particles having an average grain diameter of 5 nm gatheredfrom place to place to form large clusters. Each of these clustersconstitutes a single secondary particle. (The average grain diameter ofthe secondary particle at the location where the micrograph was takenwas about 100 nm.) The powder composed of these secondary particles hadfluidity as a whole.

The composition of the FePt nanoparticle powder was analyzed using anenergy dispersive X-ray spectrometer (TMR-EDX) and found to have anatomic ratio of Fe to Pt of 59:41. The FePt nanoparticle powder wasfurther subjected to Mossbauer spectroscopy. The result obtained isshown in FIG. 2. As can be seen from FIG. 2, a spectrum corresponding tothe ferromagnetism order of L1 ₀ ordered phase was observed at roomtemperature and the proportion of the L1 ₀ ordered phase obtained byfitting (the volumetric ratio of the ferromagnetic structure, i.e., theface-centered tetragonal structure ratio) was 52 vol. %.

The pattern obtained when the FePt nanoparticle powder was subjected toX-ray diffraction (XRD) is designated (a) in FIG. 3. The diffractionpeaks corresponding to superlattice reflections (001) and (110) seen inpattern (a) are clear evidence of the presence of face-centeredtetragonal structure.

Magnetic torque measurement carried out on the FePt nanoparticle powdershowed it to have a magnetic anisotropy Hk of 31 kOe. An attempt wasmade to measure the transition starting temperature of the FePtnanoparticle powder using a differential scanning calorimeter (DSC) butno distinct transition temperature was observed. This is thought to bebecause fct formation had already progressed to a considerable extent.

The saturation magnetization as of the FePt nanoparticle powder measuredin a magnetic field of 5T using a SQUID magnetometer was 52 emu/g.

Comparative Example 1

Example 1 was repeated, except that the circulation was continued for3.5 hr at a temperature of 260° C. As in Example 1, the obtained FePtnanoparticle powder was subjected to TEM observation, compositionanalysis by TMR (transversal microradiography), Mossbauer spectroscopy,X-ray diffraction, magnetic torque measurement and DSC measurement.

The average grain diameter of the primary particles determined by TEMobservation was 5.4 nm and the powder was found by TMR compositionanalysis to be composed of FePt nanoparticles of an atomic ratio of Feto Pt of 58:42. However, as can be seen from the results of Mossbauerspectroscopy shown in FIG. 4, no spectrum corresponding to theferromagnetism order of L1 ₀ ordered phase was observed at roomtemperature. From this it can be assumed that the proportion of the FePtnanoparticles accounted for by L1 ₀ ordered phase was 0 vol. %. And, infact, as can be seen from pattern (b) in FIG. 3, X-ray diffractionanalysis revealed no peaks corresponding to superlattice reflections(001) and (110), while a distinct transition temperature was observed byDSC. The transition starting temperature was 280° C. Magnetic anisotropyHk by magnetic torque measurement was 3.7 kOe and saturationmagnetization σs was 16 emu/g.

Comparative Example 2

Example 1 was repeated, except that ethylene glycol (boiling point:197.6° C.) was used instead of tetraethylene glycol and the circulationwas continued for 3.5 hr at a temperature of 200° C. As in Example 1,the obtained FePt nanoparticle powder was subjected to TEM observation,composition analysis by TMR, Mossbauer spectroscopy, X-ray diffraction,magnetic torque measurement and DSC measurement.

The average grain diameter of the primary particles determined by TEMobservation was 4 nm and the powder was found by TMR compositionanalysis to be composed of FePt nanoparticles of an atomic ratio of Feto Pt of 56:44. No spectrum corresponding to the ferromagnetism order ofL1 ₀ ordered phase at room temperature was observed by Mossbauerspectroscopy. From this it can be assumed that the proportion of theFePt nanoparticles accounted for by L1 ₀ ordered phase was 0 vol. %.And, in fact, X-ray diffraction analysis revealed no peaks correspondingto superlattice reflections (001) and (110). A distinct transitiontemperature was observed by DSC. The transition starting temperature was310° C. Magnetic anisotropy Hk by magnetic torque measurement was 1.7kOe and saturation magnetization σs was 5.1 emu/g.

1-7. (canceled)
 8. A magnetic metal powder having fluidity such that thepowder is free flowing, the free flowing magnetic powder having uniaxialcrystal magnetic anisotropy as produced without heat treatment and beingcomposed of magnetic metal particles whose main components and thecontents thereof are represented by the following general formula (1):[T_(X)M_(1−X)]_(Y)Z_(1−Y)  (1), where T is one or both of Fe and Co, Mis one or both of Pt and Pd, Z is at least one member selected from thegroup composed of Ag, Cu, Bi, Sb, Pb and Sn, X represents 0.3˜0.7, and Yrepresents 0.7˜1.0, the balance being impurities unavoidablyincorporated during production, which magnetic metal powder has avolumetric ratio of ferromagnetic structure (face-centered tetragonalratio) as measured by Mossbauer spectroscopy in the range of 10˜100%,saturation magnetization σs of 20 emu/g or greater, and average primaryparticle diameter by transmission electron microscopic observation (TEM)of 30 nm or less.
 9. A magnetic metal powder according to claim 8, whichhas a magnetic anisotropy Hk by magnetic torque measurement of 10.0 kOeor greater.
 10. A magnetic metal powder according to claim 8, which hasan average primary particle diameter of 20 nm or less.
 11. The powder ofclaim 8, wherein said fluidity permits the powder to rotate freely whenthe powders are positioned in and subjected to a magnetic field.